Spectrum and Its Influence on 3G and Wi-Fi Architectures

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  • 1. Spectrum and Its Influence on 3G and Wi-Fi Architectures A Technology Convergence (Triple-Play) Perspective Mark Norton Stephen Ruth George Mason University Professor of Public Policy George Mason University Summary technical aspects of Wi-Fi and CDMA plus OFDM/CDMA comparisons are included. This paper focuses on spectrum availability as the primary driver in wireless architectural design and Introduction explores the influence of spectrum policy and spectrum access techniques on the convergence The evolution of radio technology can be viewed as role of mobile communications technology and a constant effort to increase efficient spectrum uti- systems. Two wireless implementations will be lization. The first step in the evolution, spectrum reviewed: Wi-Fi (IEEE 802.11) and 3G. Both are allocation, was developed in the 1930s to control essential to convergence and triple-play strategies interference from different analog transmissions. in the United States as well as globally, since Wi- Frequency allocations were designated for a specif- Fi/Wi-MAX capabilities will need to coexist with ic purpose. Techniques such as AM/FM were 3G and its successor technologies in many parts of designed to fit within allocations based on low cost the world. After a brief description of the specifi- transmitter and receiver technology rather than cations and characteristics of Wi-Fi and 3G, the spectrum efficiency. As spectrum became more two will be compared across the following dimen- congested and digital technology emerged, the next sions in more specific terms: spectrum standards step was the development of spectrum conservation evolution, mobility versus bandwidth, cellular ver- methods. Examples of spectrum conservation sus island versus lily pond architecture, DSSS ver- methods include multilevel data transmission or sus OFDM, spectrum policy, spectrum efficiency, cellular radio zones that enable spectral reuse. The and interference. success of spectrum conservation methods has per- mitted the global access to telecommunication Conclusions and recommendations for strategic services that we enjoy today. policy shifts focus on the need to reconfigure base stations, the requirement for increased use of the In the future, communication systems will need to common model for spectrum allocation, develop- become more pervasive with even higher data ing global standards that drive economies of scale, rates. The cost and scarcity of spectrum could be a maintaining cell neighborhoods that optimally bal- major barrier to the development of the mobile ance throughput, the potential for harmonization of multimedia market. Insufficient spectrum to meet 3G/Wi-Fi services, and anticipation of a future the needs of the marketplace will result in the where ODFM/Wi-Fi compatibility will lead to access network becoming a service bottleneck. more efficient and cost-beneficial deployment of Under these circumstances, prices for market 4G systems. Two appendices describing various access would rise and service development would 271
  • 2. Spectrum and Its Influence on 3G and Wi-Fi Architectures Table 1: Multimedia Requirments be suppressed. Wireless networks were originally This is especially true of wireless systems that optimized for voice traffic. The patterns associated require approval from multiple countries, have with voice communications are well known, having legacy interoperability requirements, and cannot been observed since the invention and widespread easily be reconfigured once installed. use of the telephone. Voice traffic statistics are pre- dictable, allowing traffic engineers to use a standard Our focus is primarily on spectrum availability as methodology to estimate the amount of capacity the primary driver in wireless architectural design needed in a communications system. In contrast, and the influence of spectrum policy and spectrum there is a much wider range of requirements and access techniques on the evolution of mobile con- characteristics for wireless multimedia communica- verged networks. Tens of millions of new wireless tions than there is for voice. General multimedia users are expected to enter the market in the next requirements are listed in Table 1 and indicate several years. Spectrum access is considered key to greater breadth of performance characteristics. This improving the quality of video and data services variability prohibits data from being efficiently car- and to increase productivity through applications ried over the hierarchical networks designed for such as wireless credit processing and inventory voice traffic, whether wireline or wireless. The key management. Two specific wireless implementa- wireless difference, of course, is that high data rate tions—Wi-Fi and 3G—are particularly interesting services are constrained by spectrum access. in elaborating the spectrum access issue. We will emphasize the potential for coexistence between Mobile wireless communications have traditionally 802.11 and emerging 3G and successor (4G) posed difficult performance challenges for con- deployments. verged networks. These challenges include the vagaries of the wireless environment, the limited 3G and Wi-Fi Background applicability of wired protocols to manage conges- tion in high bit error rate wireless links, the wide This section provides a general background on 3G range of requirements and characteristics for voice, and Wi-Fi to permit a framework for additional video, and data communications, and the need for comparison. Note that other efforts to compare 3G efficient spectrum utilization. Designing for the and Wi-Fi frequently focus on the business case1 or optimum use of spectrum has become of increased do not consider a direct comparison due to the importance as spectrum planning and allocation inherent application differences.2 While there are have become protracted, expensive, and uncertain. many other emerging communications technologies 272
  • 3. Mark Norton and Stephen Ruth such as ultrawideband, Wi-MAX, etc., 3G and Wi- Comparisons between 3G and Wi-Fi Architectures Fi can be considered the two most significant tech- nologies to have gained market acceptance and will This section examines key differences between 3G define the near-term mobile communications land- and Wi-Fi wireless architectures, especially spec- scape. trum performance, interference, standards, and operating performance. This is not intended to be an 3G refers to an umbrella of standards developed as exhaustive technical analysis but an illustration of part of an International Mobile Telecommunications how technology and policy interact to influence the 2000 (IMT-2000) project designed to provide higher design of wireless systems. Appendix 2 summarizes bandwidth for digital communications. From a user’s the major differences between 3G and Wi-Fi that perspective, the key feature of 3G mobile service is are discussed in subsequent sections. that it offers (nearly) ubiquitous and continuous cov- erage. To support this service, mobile operators Mobility versus Bandwidth maintain a network of interconnected and overlap- ping mobile base stations that hand off customers as 3G and Wi-Fi provide a different range of mobile they move among adjacent cells. Each mobile base services. Local mobility, the ability to move devices station may support users up to several kilometers with cables, is one of the key advantages of WLANs away. A backhaul network provides interconnection over traditional LANs. WLANs trade the range of to the public switched telephone network (PSTN) coverage for higher bandwidth, making them more and other services and connects the cell towers to suitable for local hotspot service. 3G, on the other each other. The mobile system operator owns the hand, provides ubiquitous mobility. 3G offers much end-to-end network from the base stations to the narrower bandwidth but does so over a wider calling backhaul network to the point of interconnection to area and with more support for rapid movement the PSTN (and perhaps owns parts of the PSTN as between base stations. Although it is possible to well). Service providers are anticipated to upgrade cover a wide area with Wi-Fi, it is most commonly the existing network to support 3G standard data deployed in a local area with one or a few base sta- rates of 384 kbps up to 2 Mbps. Most commercial tions being managed as a separate WLAN. In con- applications are anticipated to offer data rates closer trast, a 3G network would include large number of to 100kbps in practice.3 Appendix 1 describes key 3G base stations operating over a wide area as an inte- specifications. grated wireless network to enable load-sharing and uninterrupted hand-offs when subscribers move Short for wireless fidelity, Wi-Fi is used generically between base stations at high speeds. Figure 1 when referring to any type of 802.11 network, shows the performance space for mobility versus whether 802.11b, 802.11a, dual-band, etc. The term bandwidth between 3G and Wi-Fi. Wi-Fi is not is promulgated by the Wi-Fi Alliance.4 Since the designed for the high mobility/car traffic scenario. 1980s, the IEEE has approved a series of Ethernet The small “cell” sizes of Wi-Fi systems would standards to support higher capacity LANs over a require unacceptably high hand-off rates. System diverse array of media. The 802.11x family of efficiency would be degraded by dedicated hand-off Ethernet standards is for wireless LANs.5 WLANs control functions rather than communication. are typically deployed in a distributed way to offer approximately 100-meter connectivity to a wireline Comparing Cellular, Lilypond, and Island backbone network. It is possible to provide contigu- Architectures ous coverage over a wider area by using multiple base stations. Still, the WLAN technology was not Three architectures—cellular, lilypond, and designed to support hand-offs associated with users island—can be considered for 3G and/or Wi-Fi moving between base station coverage areas (i.e., the deployment. Figure 2 displays a rough picture problem addressed by 3G mobile communication outline of the three models. The cellular architec- systems). In contrast to mobile, WLANs were princi- ture represents the classic mobile communication pally focused on supporting data communications. concept which employs spatial frequency re-use techniques on an interference-limited basis. This 273
  • 4. Spectrum and Its Influence on 3G and Wi-Fi Architectures Figure 1: Mobile Wireless System Landscape is achieved by dividing the service areas into cost Wi-Fi components created the option of an smaller cells, ideally with no gaps or overlaps, individual establishing his own version of a cell each cell being served by its own base station and tower. Lilypond networks use the existing Internet a set of frequency channels. The power transmit- infrastructure to aggregate independent WLANs. ted by each station is controlled in such a way that Individuals are motivated to participate in the ad the local mobile stations in the cell are served hoc network structure by a series of cost incentives: while co-channel interference in the cells using WLAN hardware technology embedded (at low the same set of radio channel frequencies is kept cost) in personnel computers/handheld devices, acceptably minimal. An added characteristic fea- Internet service providers (ISPs) providing low cost ture of a cellular system is its ability to adjust to data access rates, etc. the increasing traffic demands through cell split- ting. The basic business model is the telecommu- Interference mitigation in lilypond networks is con- nications services model in which service ducted at each access point via a set of dynamic providers own and manage the infrastructure technical options that include low power emission, (including the spectrum) and sell service on the selection of different frequency bands, and an infrastructure. In this architecture, Wi-Fi systems inherent assumption that owners will tolerate are viewed as independent extensions of wired reduced data rates in times of congestion. This sce- data networks providing un-tethered data access nario is considered most viable for urban settings for indoor office/home applications. that integrate indoor hotspots such as hotels and air- ports. Note that the lilypond scenario has not been Nicholas Negroponte of MIT has suggested that implemented yet and represents an ideal—the direct nationwide telecommunication networks that were opposite of the centralized cellular model. created in a top-down manner are about to be replaced by a “lily pond” patchwork of Wi-Fi sta- The island, or hotspot, model attempts a symbiotic tions created by generous individuals, entrepre- relationship between Wi-Fi access technology and neurs, and public/private institutions from the bot- 3G by coupling the WLAN (when needed) with 3G tom-up. He referred to this as “a broadband system services. Integrating 3G and Wi-Fi networks pro- built by the people for the people.”6 vides the opportunity to offer good voice/data telephony support while providing high data rate The wide-scale implementation of telecommunica- island connectivity in high demand areas, like air- tion lilypond was not considered feasible until low ports. This is a hybrid of the cellular and lilypond 274
  • 5. Mark Norton and Stephen Ruth Figure 2: Different Mobile Communication Architectures 31, 32, 33 model and is frequency described as the “conver- modulation can be expressed in a discrete frequen- gence approach” by industry representatives.7 cy domain after going through a transformation. It is considered a power-efficient modulation in which DSSS versus OFDM less spillover energy is outside the bandwidth.8 OFDM offers multiple signal processing benefits One of the key differences between Wi-Fi and 3G is that have not been available in 3G modulation the radio access method. Wi-Fi/802.11a uses methods, and it possesses a number of unique fea- OFDM (orthogonal frequency division multiplex- tures 9, 10 that make it an attractive choice for high- ing) as opposed to the usual DSSS (direct sequence speed broadband wireless communications: spread spectrum) method used by 3G and 802.11b. These access methods have inherent differences • OFDM is considered robust against multipath since DSSS (used in CDMA, wideband CDMA, fading and intersymbol interference because the and cdma2000) forces a multiplication of the signal symbol duration increases for the lower-rate in the time domain that spreads it in the frequency parallel subcarriers. domain while OFDM uses a forced multiplication in the frequency domain to spread the symbol in the • OFDM allows for an efficient use of the avail- time domain. There is a major debate in the indus- able radio frequency (RF) spectrum through the try as to whether OFDM has inherent advantages use of adaptive modulation and power alloca- over CDMA in cellular networks. IEEE 802.16 (as tion across the subcarriers that are matched to supported by the Wi-MAX Forum) has chosen slowly varying channel condition. OFDM as the basis of its radio technology. OFDM is also used in IEEE 802.11a, IEEE 802.11g, and • Unlike other competing broadband access tech- IEEE 802.11n. nologies, OFDM does not require contiguous bandwidth for operation. OFDM is a modulation technique where informa- tion symbols are transmitted in parallel by applying • OFDM makes single-frequency networks pos- them to a large number of orthogonal subcarriers. sible, which is particularly attractive for broad- The attractive advantage of using OFDM is that the casting applications. 275
  • 6. Spectrum and Its Influence on 3G and Wi-Fi Architectures OFDM’s advantages may not be significant when how spectrum standards have evolved to create Wi- compared to 5MHz 3G since data rates may not be Fi economies of scale; the second discusses spec- limited by intersymbol interference. But with next trum efficiency comparisons between Wi-Fi and 3G generation systems that will deliver throughput systems. rates in the 10 Mbps to 100 Mbps range, the advan- tages could become significant. Third Generation The Evolution of Spectrum Standards Partnership Project (3GPP) has a work group study- Different methods of standard evolution character- ing how current cellular technologies might best ize 3G and Wi-Fi development. Global Wi-Fi stan- evolve, including looking at OFDM. OFDM, how- dards are coordinated via an IEEE process and have ever, is not the only option. Another viable possibil- developed a single standard. This is in contrast to ity is multicarrier CDMA where multiple CDMA the multiple 3G standards develop by industry con- channels are combined for higher throughputs. sortia. Appendix 1 describes the technical character- istics of the three 3G standards and compares it with Spectrum Wi-Fi. The simple observation can be made that perform- 3G “standards” have so many mutually exclusive ance is anticipated to favor lower 3G spectrum options that all the 3G systems launched so far can since free-space propagation path loss increases claim compliance with it, yet none actually interop- with frequency. Note however that this may be par- erate with each other. An example is the failure of the tially offset by practical antenna gain which also is 3G spectral harmonization effort which was initiated frequency-dependent. The different spectrum poli- in 1992 with the goal of a global standard. After cies that govern 3G and Wi-Fi establish the founda- years of debate a compromise was accepted that per- tion for many of the technical differences between mitted operation in both 1.885–2.025 GHz and 3G and Wi-Fi.11 Consider, for example, the regula- 2.110–2.2 GHz.14 By that time, the vision of a global tions governing 1.9 GHz PCS and 2.4 GHz indus- standard was discarded as competing technology— trial, scientific, and medical (ISM) bands. The ISM wideband code division multiple access (CDMA) in band power limit in the United States is 4 watts Japan and CDMA 2000 in the United States—require compared with the PCS base station limit of 1600 different spectrum allocations.15 Efforts to extend 3G Watts. Consequently, the licensed band system has spectrum continue as the United States prepares to a huge advantage in the downlink power budget. allocate an additional 90 MHz. The Commission The downlink power advantage is precisely what intends to commence an auction for advanced wire- enables the licensed CPE (customer premises less services licenses in the 1710–1755 MHz and equipment) device to work indoors without the 2110–2155 MHz bands as early as June 2006. These need for an outdoor, elevated, near-line-of-sight, bands represent a portion of the spectrum identified high-gain receive antenna.12 at the World Radiocommunication Conference (WRC-2000) for 3G wireless use. The main difference between cellular and Wi-Fi is that the cellular system uses the licensed spectrum In contrast to 3G, Wi-Fi has been able to develop a and Wi-Fi is implemented in unlicensed bands. The global spectrum standard. During the late 1990s, economic basis for its implementation is therefore wireless local-area networks, notably the IEEE completely different. The success of Wi-Fi has 802.11 family of standards, known as Wi-Fi, and made many people look to the unlicensed spectrum the European HIPERLAN system, became popular as the future of wireless access, rather than spec- as broadband wireless systems. They were based on trum licensed and controlled by large corporations. projected needs for additional spectrum beyond the The fact that Wi-Fi and 3G use different spectrum previously defined bands in the 2.4-GHz band pro- allocation models13 creates distinct technical design ponents requested and received an additional 355 objectives for cost of service and congestion man- MHz of spectrum in the 5-GHz region.16 The Part agement. The following sections review two specif- 15 rules were amended in 1998 to provide for oper- ic areas where spectrum has had an impact on wire- ation of unlicensed national information infrastruc- less systems design objective. The first examines ture (U-NII) devices in the 5-GHz band (5.15–5.35 276
  • 7. Mark Norton and Stephen Ruth GHz and 5.725–5.825 GHz). The FCC recognized certain services and technologies and disadvantages that developments in a number of different digital to others. Rough estimates of spectrum efficiency are technologies had greatly increased the need to considered useful in certain situations since they transfer large amounts of data from one network or allow for some comparisons between technologies. system to another. In making this spectrum avail- able, the FCC concluded that providing additional General observations concerning Wi-Fi- versus 3G- spectrum for unlicensed wideband operation would efficiency favor Wi-Fi due to fundamental range benefit a vast number of users, including medical, (small diameter cell) scenario assumptions: educational, and business/industrial users.17 • Lee20 concluded that C/I (carrier-to-interfer- To support growing interest in real-time services ence) of a wireless system under a non-fading such as voice and video over IP networks, multiple (considered fixed-to-fixed) condition is always new 802.11 standards are under development by the less then the required C/I ratio of a cellular sys- IEEE.18 These new standards will provide a range of tem under a mobile multipath fading condition. new applications, spectrum management, and inter- operability with cellular systems: • Hammada21 describes how the efficiency (chan- nels/Mhz/km2) of a given modulation technique • IEEE 802.11j: Japanese regulatory extensions is inversely proportional to the “cell” or coverage • IEEE 802.11k: Radio resource measurements area. Wi-Fi, with its small coverage areas, is thus • IEEE 802.11n: Higher throughput improve- anticipated to always outperform 3G systems. ments via MIMO techniques This general rule does not take into account, how- • IEEE 802.11p: WAVE, wireless access for the ever, factors which may reduce efficiency, such vehicular environment as additional control channels to account for a • IEEE 802.11r: Fast roaming high hand-off rate due to small cell size. • IEEE 802.11s: Wireless mesh networking • IEEE 802.11u: Interworking with non-802 net- Another approach to compare OFDM and CDMA is works (e.g., cellular) to establish a level playing field where each technol- • IEEE 802.11v: Wireless network management ogy is compared against the same operating con- straints. This was done by Lawrey22 for a cell phone Spectrum Efficiency model (see Appendix 2). OFDM was found to per- The ideal spectrum metric would provide a compre- form very well compared with CDMA, and it out- hensive measure of how efficient the system is, performed CDMA in many areas for a single and regardless of the modulation and multiple access multicell environment. OFDM was found to allow techniques employed. Such a measure should also be up to 2–10 times more users than CDMA in a single independent of the technology implemented with an cell environment and from 0.7–4 times more users in allowance for new techniques that may improve the a multicellular environment. The difference in user spectral efficiency and/or system quality. Objective capacity between OFDM and CDMA was dependent spectral efficiency measures are useful for establish- on whether cell sectorization and voice activity ing performance benchmarks, estimating ultimate detection is used. OFDM would require a frequency performance capacity, and establishing minimum reuse pattern to be used in a multicellular environ- spectral efficiency standards for regulation. ment to reduce the level of intercellular interference. The general conclusion is that OFDM has compara- Despite the intended value of spectrum efficiency ble spectral efficiency performance to CDMA and standards, the current communication environment that small cell sizes are critical to providing reduced often makes establishing objective standards a fruit- multipath and high teledensity environments. less task. The FCC Spectrum Task Force19 recently concluded that it is not appropriate to select a single, Interference objective metric that could be used to compare effi- ciencies across different radio services. Inherent in Unlike 3G systems, Wi-Fi systems must adapt to a the assumptions of any metric would be advantages to variety of interactions within a particular electro- 277
  • 8. Spectrum and Its Influence on 3G and Wi-Fi Architectures magnetic environment. 802.11b systems are more point to reduce transmission power in order to susceptible to radio frequency interference than reduce interference with other users. Dynamic man- 802.11a because of many sources of interference in agement techniques such as DFS may enable the the 2.4 GHz band. Microwave ovens, Bluetooth management of spectrum in real time. These sys- devices, 2.4-GHz cordless phones, and other nearby tems first identify what spectrum is available and 802.11 wireless LANs can cause significant and then assign it to users for the best benefit. Such damaging (radio frequency) interference with opportunistic use of the spectrum may ultimately 802.11b systems. Most brands of cordless phones provide the highest values for spectral efficiency can bring a wireless LAN to a standstill while the over given time period and geography.23 phones are in use within 75 feet of 802.11 devices. Radio frequency interference with 802.11a in the 5 Synthesis and Conclusions GHz band is less common even though there a vari- ety of radar and satellite communications that share This paper focused on spectrum availability as the the band on a co-primary basis. primary driver in wireless architectural design and explored the influence of spectrum policy and spec- 802.11a and 802.11b use a carrier-sensing protocol trum access techniques on the evolution of mobile that enables the sharing of a common radio chan- converged networks. The ability to guarantee spec- nel. An end user’s radio network interface card trum availability is becoming a critical driver in (NIC) senses the air medium and only transmits if wireless system design and has broad implications no radio frequency waves above a certain threshold for global market development, localized network are detected. The presence of a radio frequency sig- performance, and requirements for new communi- nal causes the radio NIC to hold off from transmit- cation technologies. ting, and an interfering signal strong enough will make the channel appear as busy. As a result, the Perhaps the most significant observation is that radio NIC will wait until the interference goes spectrum engineering must be conducted at the away (which could be minutes, hours, or days). global level. Technical differences often are driv- Radio frequency interference competes with the en by spectrum policy and the politics involved in transmission of 802.11 frames and can significant- creating global standards. The harmonization of ly reduce the throughput and availability of a wire- the 5 GHz spectrum, which came about as a result less LAN. of the World Radiocommunications Conference and ratification of IEEE 802.11h—a standard to The 802.11a standard supports much higher data reduce interference in the 5 GHz band—stands as rates on a channel. DSSS (similar to CDMA) basi- a compelling example. Together, these two cally uses a single carrier, and it is subject to “sin- achievements will allow products that utilize the 5 gle point failures” where a fading trough is encoun- GHz band to achieve the same economies of scale tered on that carrier. OFDM further spreads data that aided the success of earlier products in the across multiple sub-carriers in the allocated band, 2.4 GHz band. Previously, manufacturers had to and allows redundant coding to recover data in a create an unwieldy array of products to match a noisy environment, because it is unlikely that all patchwork environment. Now they will be able to sub-carriers will experience an identical fading concentrate more efficiently on one standards- trough simultaneously. This mechanism is less like- based solution. ly to have a “single point failure.” 3G “standards” have so many mutually exclusive The Wi-Fi industry long-term approach to interfer- options that all the 3G systems launched so far can ence mitigation relies on upgrades to the 802 stan- claim compliance with it, yet none actually interop- dard to permit dynamic spectrum management. erate with each other. Wi-Fi standards, on the other Dynamic frequency selection (DFS) allows devices hand, are established through an IEEE process to detect such transmissions and switch to an alter- which has a more evolutionary approach and, when native channel. Additionally, a transmit power con- it is accepted, will achieve the objective of a true trol protocol will allow users close to an access worldwide standard. 278
  • 9. Mark Norton and Stephen Ruth Another important observation is that high data rate New communications technologies are providing triple-play services are inherently a local phenome- opportunities to obtain more services and capacity na. As data rates increase, physics dictates that from spectrum. Despite the universal 3G accept- small coverage areas provide the greatest efficien- ance of CDMA, OFDM may become the lead tech- cy. 3G base stations will need to be packed tighter nology for 4G25 networks. While there are many to be competitive with data rates. Small area net- technical issues with adapting OFDM for a large works can be tailored to support increased link pre- scale cell phone network,26 Wi-Fi OFDM has dictability and higher teledensities due to the changed the landscape by creating an OFDM indus- reduced multipath, improved propagation pre- try base, an OFDM Wi-Fi interoperability require- dictability, and higher link margins. Current 3G ment, and an alternate approach to achieving spec- metropolitan networks and Wi-Fi local area net- trum efficiency. Figure 3 depicts the influence of works may represent the first steps toward an inter- Wi-Fi on 3G standards; the arrows are designed to mediate neighborhood area network (NAN)24 archi- show the influence of spectrum on the design on tecture. NANs represent a new architectural system future wireless systems. for broadband wireless local distribution that pro- vides services tailored to local user applications, Wi-Fi’s greatest long-term implication is that it may interfaces with wired facilities, and permits auto- establish the benefit of alternate communication matic radio resource spectrum management. options—not only specific technologies such as OFDM but substantiation of the commons models Successful wireless systems must simultaneously as prudent government spectrum allocation policy. satisfy global standards that drive economies of The model of licensed “exclusive” spectrum pro- scale while maintaining small cell characteristics viding optimum quality of service and government that optimally balance throughput to mimic wired revenue is under challenge by the unlicensed com- performance. We refer to this as the global neigh- mon spectrum allocation model. Since Wi-Fi essen- borhood requirement for wireless design. Global tially has “free spectrum” while 3G must pay a standards optimized for local neighborhood per- spectrum “tax,” Wi-Fi has an incredible advantage. formance establishes the foundation for new tech- Wi-Fi systems have the benefit of large bandwidth nologies and wireless services. if they can control interference. A no-cost market entrance provides great incentive to mitigate the Figure 3: Mobile System ‘Possible’ Evolution – Wi-Fi Impact 279
  • 10. Spectrum and Its Influence on 3G and Wi-Fi Architectures inherent problems involved in spectrum sharing. On MHz. The OFDM system could handle 64 users the other hand, by paying for spectrum, 3G systems each at 39 kbps, or 128 users at 19.5 kbps depend- buy a level of interference that permit the optimiza- ing on the spectrum allocation. The figure of merit tion (spectrum efficiency) of resources across a derived was 2-4/bits/sec/hz depending on BER. broad geographic region. Rappaport derives the capacity of a single cell At just a 300-foot radius, Wi-Fi is clearly a local- CDMA system to be area network. Outside of densely populated urban areas, true mobile service is impractical. A wider, albeit slower network, such as 3G, will be neces- sary to fill the “white space” of Wi-Fi networks. However, Wi-Fi will provide cost-effective data services for slow moving mobile subscribers that where are comfortable with roaming for Wi-Fi access G is the antenna sectorization, 2,55; points. This market segmentation has the potential d is the voice duty cycle, .4; to usurp some of the expected revenues from 3G Eb/No is the energy per bit to noise ratio, 8dB; networks. W is the total transmission bandwidth,1.25 MHz; R is the base band bit rate,19.5KHz; Perhaps the most likely outcome is that we will live n/S is the ratio of received thermal noise to user sig- in a world where 3G and Wi-Fi work together, with nal power(assuming no thermal noise). recognition that they are complementary, not com- peting, technologies.27 Wi-Fi has a potential for low-cost networks that grow organically to serve communities of users. A number of obstacles (such as poor mobility management/roaming capability, patchy interoperability, and interference problems) The spectral efficiency is thus must be overcome so that Wi-Fi can be competitive with 3G and Wi-Fi service can be guaranteed. Handsets and computers are being developed which will allow roaming between the two networks. This will allow 3G to take advantage of Wi-Fi’s increased bandwidth in city hotspots, with 3G pro- This is roughly half the capacity of the OFDM sys- viding coverage elsewhere. tem: 2.1bits/sec/Hz. Appendix 1: Wireless Wi-Fi versus 3G Data28 OFDM was found to perform very well compared with CDMA, with it out-performing CDMA in See Table 2. many areas for a single and multicell environment. OFDM was found to allow up to two to 10 times Appendix 2: Comparison of CDMA versus OFDM more users than CDMA in a single cell environ- with Multiple Cells ment and from 0.7 to four times more users in a multicellular environment. The difference in user A capacity comparison of CDMA versus OFDM capacity between OFDM and CDMA was depend- with multiple cells is provided for cellphone net- ent on whether cell sectorization and voice activi- works to illustrate the efficiency advantage of ty detection is used. OFDM would require a fre- OFDM. Data, formulas, and assumptions devel- quency reuse pattern to be used in a multicellular oped to model OFDM (from Lawrey29) have been environment to reduce the level of intercellular extended to CDMA (from Rappaport).30 interference. OFDM Calculations and Assumption The OFDM example used a bandwidth of 1.25 280
  • 11. Mark Norton and Stephen Ruth 5. For more information see http://grouper.ieee.org/ References groups/802/index.html. 6. N. Negroponte, “Being Wireless,” Wired, Issue 10.10 - 1. W. Lehr and L. McNight, “Wireless Internet Access: 3G vs Oct. 2002, http://www.wired.com/wired/archive/10.10/ WiFi,” http://itc.mit.edu/itel/docs/2002/LehrMcKnight_ wireless.html WiFi_vs_3G.pdf 7. See May 4, 2003, from Shorecliff Communication, LCC 2. J. Gilbert, “3G and WLAN – Friend or Foe? (a.k.a. why “Returning to Basics—Leveraging Convergence 3G sucks),” Atheros Communications, BWRC Winter Opportunities,” http://www.shorecliffcommunications.com/ Retreat, January 7, 2002. bwwf03/default.asp?showid=B038&info=523 3. M. Martin, “Verizon Wireless Gets Closer to 3G,” Network 8. W. Lee, “Lee’s Essentials of Wireless Communications,” World, February 2, 2002. McGraw-Hill, 2001. 4. Wi-Fi Alliance, http://www.weca.net/OpenSection/ index.asp Table 2 281
  • 12. Spectrum and Its Influence on 3G and Wi-Fi Architectures 9. J. Chuang and N. Sollenberger, “Beyond 3G: Wideband 23. J. Chen and J. Gilbert, “Measured Performance of 5-GHz Wireless Data Access Based on OFDM and Dynamic 802.11a Wireless LAN Systems,” Atheros Packet Assignment,” IEEE Communications, 2000. Communications, 2001. 10. T. Rappaport et al., “Wireless Communications: Past 24. 4G, Neighborhood Area Networks, IEEE 802.11- Events and a Future Perspective,” IEEE Communications, 05/0173r0, http://grouper.ieee.org/groups/802/802_ May 2002. tutorials/march05/11-05-0173-00-0000-4g-neighborhood- 11. T. S. Rappaport, “The future growth of consumer based area-networks.ppt mobile and portable communications systems will be tied 25. See The Book of Vision 2001 – Vision of the Wireless more closely to radio spectrum allocations and regulatory World, Version 1.0 WWRF, December 2001, decisions which affect or support new or extended services http://www.wireless-world-research.org […],” Wireless Communications, Second Ed.,. p. 2, 2002. 26. Multicarrier systems are inherently more susceptible to 12. http://www.shorecliffcommunications.com/magazine/vol- frequency offset and phase noise. Frequency jitter and ume.asp?Vol=34&story=332 Doppler shift between the transmitter and receiver causes 13. Proposed by the FCC Spectrum Policy Task Force in intercarrier interference (ICI), which degrades the system November 2002 for the management of spectrum, the performance unless appropriate compensation techniques “command-and-control” model, the “exclusive use” are implemented. See J. Chaung and N. Sollenberger, model, and the “commons” model. “Beyond #G: Wideband Wireless Data Access Based on 14. WRC-2000 modified RR footnote 5.388 allocating 1.885 OFDM and Dynamic Packet Assignment,” IEEE -2.025 GHz and 2.110-2.2 GHz for 3G. 2.5 GHz has not Communications, July 2000. been allocated for 3G to this date. WRC-2007 may 27. T. Bresien, “WLAN Billing App Signal New Industry address 2.5 GHz allocation. Trend?” April 15, 2003, http://www.wirelessnewsfactor 15. “Spectrum Wars” .com/perl/story/21269.html 16. P. Bahl, “Spectrum Policy: Property or Commons? 28. Information from ITU-R Recommendation 1457 “detailed Spectrum Etiquette’s for Short Range Wireless Devices specifications of the radio interfaces of international Operating in the Unlicensed Band – A Proposal,” Stanford mobile telecommunications–2000 (IMT–2000)” Law School, March 2, 2003, http://cyberlaw.stanford.edu/ 29. E. Lawrey, “The suitability of OFDM as a modulation spectrum techniques for wireless less telecommunication,” 1997. 17. T. Cokenias, New Rules for Unlicensed Digital 30. T. S. Rappaport, Wireless Communications Principles & Transmission Systems, Compliance Engineering 2002 Practice, IEEE Press, Prentice Hall, N.Y., pp. 169–177, Annual reference Guide, www.ce-mag.com/archive/02/ 1996. Spring/cokenias.html 31. Chart modified from Uf Tureli Stevens Ins. of Tech., Aug 18. http://grouper.ieee.org/groups/802/11/ 31, 2004. Lecture Notes, Multicarrier Communications 19 Federal Communications Commission Spectrum Policy Lecture 1, http://koala.ece.stevens-tech.edu/~utureli/ Task Force Report of the Spectrum Efficiency Working EE615/lecture/F2004/lecture1.ppt Group November 15, 2002, http://www.fcc.gov/sptf/files/ 32. Island model adapted from Motorola presentation to the SEWGFinalReport_1.pdf FCC Technology Advisory Council, December 5, 2001. 20. W. C.Y. Lee, Mobile Communications Engineering, 33. Cellular Grid graphic obtained from Andrea Goldsmith, McGraw-Hill, pp. 638–646, 1997. Standford University Lecture Notes, Advanced Topics in 21. H. Hammuda, Cellular Mobile Radio Systems, John Wilet Wireless Communications, Spring 2004, Lecture 9, and Sons, pp. 74–76. www.standford.edu/c;ass/ee360/lecture9.ppt 22. E. Lawrey, “The suitability of OFDM as a modulation techniques for wireless less telecommunication,” 1997. 282